1. Field of the Invention
The present invention relates to a method of determining routes for a plurality of wiring connections on a circuit board. It is particularly concerned with circuit boards having a plurality of layers. The present invention also relates to a circuit board having wiring connections with routes determined by such a method.
2. Summary of the Prior Art
When electronic components, such as semiconductor chips and other electronic devices are mounted on a region of a circuit board, it is necessary to interconnect terminals of such components. For this purpose, wiring connections (also referred to herein as "nets") are formed on the surface of one or more layers of the circuit board in a suitable wiring region. The wiring connections then define the electrical paths for the interconnections of the devices mounted on the circuit board.
It is common for the pattern of the wiring connections to be extremely complicated, and it is not practical to define the path of each wiring connection independently. Instead, rules must be established for determining the pattern of wiring connections, i.e. constraints on the path of the wiring connections.
Normally, the or each layer of the circuit board has a "preferred" direction, i.e. a direction in which the wiring connections should extend over the majority of its length. It is normally not essential for each wiring connection to extend in the preferential direction over its entire length, and parts of the wiring connection may extend in directions other than the preferential direction. However, such parts should be minimized in order to reduce the complexity of the pattern of the wiring connections.
The difficulty of determining the pattern of wiring connections is increased when the circuit board has a plurality of layers, which are normally parallel. Then, wiring connections on respective layers are interconnected by via holes which extend through the circuit board. In such a multi-layer circuit board, it is necessary to define the preferred direction of wiring connections for each layer.
In determining the pattern of wiring connections on the or each layer, there will be restrictions on the number of wiring connections that can pass through any given area of the circuit board, because a gap must be maintained between the wiring connections to prevent electrical contact or cross-talk therebetween. Hence, when determining the pattern of wiring connections, note must be taken of areas of congestion of the wiring connections. In such areas of congestion, the number of wiring connections approaches the maximum number permitted, as determined by the minimum width of the wiring connections, and the minimum spacing therebetween.
When determining the pattern of wiring connections in a multi-layer circuit board, the standard known method is to use a technique called the "xy rule". This rule proposes that alternate layers of the circuit board have preferred wiring directions which are perpendicular. Thus, if the preferred direction of one layer is e.g. parallel to one side of the circuit board, the preferred direction for the or each adjacent layer is parallel to a perpendicular side of the circuit board.
The use of the xy rule is common because it permits a relatively ordered pattern of wiring connections. However, it has been recognized that use of the xy rule is not wholly satisfactory, particularly for patterns of wiring connections with high density. In such cases, it is difficult to avoid congestion of the wiring connections in one or more parts of one or more layers of the circuit board, because the xy rule itself imposes a limit on the patterns of wiring connections that are possible.
Therefore, proposals have been made for arrangements in which the xy rule is not applied. In JP-A-54-79472, it was proposed that a multi-layer circuit board had some layers with a common preferred direction for the wiring connections, and the other layers for which the preferred direction was orthogonal. Then, patterns could be designed in which the majority of the wiring connections extended in the direction corresponding to the majority of the layers.
Furthermore, JP-A-2-127772 proposed optionally, that, one layer of a two-layer circuit board be divided into two or more areas, and each area had a preferred direction which was inclined to the normal orthogonal directions determined by the xy rule, and the other layer be un-divided and have a preferred direction which is one of the normal orthogonal directions. Thus, this disclosure proposed that there was one or more additional preferred directions, in addition to the two orthogonal preferred directions established by the xy rule.
In JP-A-2-90368, it was proposed that on full surface-mounted electronic devices, the preferred wiring direction was perpendicular to the adjacent side of the surface-mounted electronic devices.
Finally, in an article entitled "An Iterative-Improvement Penalty-Function Driven Routing System" by R. L. Linsker, in IBM J. Res. Develop., Vol. 28, No. 5, pages 613-624, published in 1984, it was proposed that an iterature process was carried out, in which the xy rule was ignored, and the pattern of wiring connections was built up by successive iterations.